[3] Plasma osmolarity of some reptiles, especial those from a freshwater aquatic environment, may be lower than that of mammals (e.g. < 260 mOsm/L) during favourable conditions.
Consequently, solutions osmotically balanced for mammals (e.g., 0.9% normal saline) are likely to be mildly hypertonic for such animals.
[citation needed] Deep-sea fish have adapted to the extreme hydrostatic pressures of depth through a number of factors, including increasing osmolality, with one of the deepest known fish in the world, the hadal snailfish (Notoliparis kermadecensis) having a recorded muscle osmolality of 991 ± 22 mOsmol/kg, almost four times the osmolality of mammals and three times that of shallow water fish species (typically 350 mOsmol/kg).
If the ECF were to become too hypotonic, water would readily fill surrounding cells, increasing their volume and potentially lysing them (cytolysis).
In normal people, increased osmolality in the blood will stimulate secretion of antidiuretic hormone (ADH).
A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.
This ADH secretion may occur in excessive amounts from the posterior pituitary gland, or from ectopic sources such as small-cell carcinoma of the lung.
According to the international SI unit use the following equation : Calculated osmolarity = 2 Na + Glucose + Urea (all in mmol/L) As Na+ is the major extracellular cation, the sum of osmolarity of all other anions can be assumed to be equal to natremia, hence [Na+]x2 ≈ [Na+] + [anions] To calculate plasma osmolality use the following equation (typical in the US): If the patient has ingested ethanol, the ethanol level should be included in the calculated osmolarity: Based on the molecular weight of ethanol the divisor should be 4.6 but empiric data shows that ethanol does not behave as an ideal osmole.